1. ASCO WG on ozone cross sections Ground based measurements of total ozone Alkis Bais (on behalf of the ASCO WG) 9/26/2013COST ES12017, Manchester, 26-27 September 2013

2. What is ASCO? “Absorption Cross Sections of Ozone” Ad-hoc committee under WMO and IO 3 C To investigate and recommend ozone absorption cross sections for ground-based ozone retrievals Final meeting of ASCO in Geneva, June 2013 Report to SAG Ozone for decision 9/26/2013COST ES12017, Manchester, 26-27 September 2013

3. O3 X-section datasets Bass and Paur (1984) – PB – (6 temperatures, 203K – 298K) Brion, Daumont, Malicet (1995) - BDM – (4 (+1) temperatures, 218K -295K) Serdyuchenko (2012) – IUP – (11 temperatures, 193K -293K) 9/26/2013COST ES12017, Manchester, 26-27 September 2013

4. Temperature sensitivity 9/26/2013COST ES12017, Manchester, 26-27 September 2013

5. 9/26/2013COST ES12017, Manchester, 26-27 September 2013

6. Dobson: – Ozone retrieval is almost independent of XS, with absolute differences of the order of 1% or less – IUP XS improve the agreement between CD and AD pair ozone – Probably it is not worth to reprocess the whole ozone database for such small changes, unless temperature dependent XS are taken into account. Brewer: – DBM produces differences of up to 3% compare to BP – UIP has a small effect (~1%) on ozone and leads to smaller differences with the Dobson – DBM and IUP reduce the seasonality between Dobson and Brewer 9/26/2013COST ES12017, Manchester, 26-27 September 2013

7. IUP X-sections Spectral noise (temperature dependent) 9/26/2013COST ES12017, Manchester, 26-27 September 2013

8. (283, 273, 253, 233, 213, 203 K) Temperature dependence of IUP 9/26/2013COST ES12017, Manchester, 26-27 September 2013

9. 9/26/2013COST ES12017, Manchester, 26-27 September 2013 Fits on slit-convoluted data

10. Temperature dependence of IUP cross sections: – Quadratic approximations that are used to derive T- dependent XS spectra introduce uncertainties. – The Temperature dependence of the XS changes when different combinations of temperatures are used. – These quadratic approximations are used also as inputs in models (indirect effects through the use of modeling results in the retrievals). Conclusion: The IUP XS at least in the spectral region used by the GB instruments must be standardized. 9/26/2013COST ES12017, Manchester, 26-27 September 2013

11. Action item: Look closer to the temperature dependence of IUP. The laboratory groups must provide a “clean” dataset at least for the spectral regions of interest (e.g. 300-330 nm). Action Item: An error budget should be produced for the ozone retrieval taking into account the uncertainties reported from the lab groups (spectroscopic data, parameterizations, etc.) – Similar studies have done in the past by: a) Basher for the Dobson (WMO report), and b) Cede for the Brewer(unpublished) – Possibility this action item to be accomplished in the framework of the COST action EUBREWNET 9/26/2013COST ES12017, Manchester, 26-27 September 2013

12. Effective temperature dependent IUP cross sections – Implementation in Dobson will remove the seasonality. – Seems not very important for Brewers (with current quadratic fits) Action item: What kind of accuracy is needed for the calculation of the effective temperature? – Different sources of ozone and temperature profiles. Climatology, NCEP, ECMWF, radiosondes, etc. Implications: Difficulties in reprocessing the whole ozone database. In some cases other factors are more important in ozone retrievals than the choice of the XS. (e.g. large airmass factors at high latitudes in conjunctions with spatial variability of stratospheric ozone) 9/26/2013COST ES12017, Manchester, 26-27 September 2013

13. Temperature dependent X-sections and ozone retrieval Effective temperature dependent IUP cross sections – Implementation in Dobson will remove the seasonality. – Seems not very important for Brewers (with current quadratic fits) Action item: What kind of accuracy is needed for the calculation of the effective temperature? – Different sources of ozone and temperature profiles. Climatology, NCEP, ECMWF, radiosondes, etc. Implications: Difficulties in reprocessing the whole ozone database. In some cases other factors are more important in ozone retrievals than the choice of the XS. (e.g. large airmass factors at high latitudes in conjunctions with spatial variability of stratospheric ozone) 9/26/2013COST ES12017, Manchester, 26-27 September 2013

14. IUP is recommended because: Dobson absorption coefficients will remain the same Brewer absorption coefficient will change by a small amount and towards the proper direction to improve the comparison of with and Dobson Including T-dependent coefficients increases the accuracy of the ozone retrieval Implementation will be decided by SAG Ozone 9/26/2013COST ES12017, Manchester, 26-27 September 2013